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Abstract

We study a coupled cavity laser configuration where a passively Q-switched Nd:YAG microchip laser is combined with an extended cavity, including a doped multimode fiber. For appropriate coupling levels with the extended cavity, we observed that beam self-cleaning was induced in the multimode fiber thanks to nonlinear modal coupling, leading to a quasi-single mode laser output. In the regime of beam self-cleaning, laser pulse duration was reduced from 525 to 225 ps. We also observed a Q-switched mode-locked operation, where spatial self-cleaning was accompanied by far-detuned nonlinear frequency conversion in the active multimode fiber.

Figures (8)

Fig. 1 Schematic of the coupled cavity composite MMF laser comprising a Nd:YAG/Cr:YAG microchip, half-wave plates (HWP), a polarizing beam splitter (PBS), a pump laser diode (LD) coupled to the 80 cm long Yb-MMF by a dichroic beam splitter (DBS) and a highly reflective mirror (Mext). The PBS provides also two of the output ports of the laser, indicated as (1) and (2); a third access point is represented by the weak transmission of mirror Mext Inset: refractive index profile of the fiber core (dark line) compared to a theoretical parabolic profile (Red dashed line).

Fig. 4 Intra-cavity average power measured in the MMF branch (red solid dots) for various level of coupling with the microchip cavity, as set by the orientation of the HWP#1. The red dashed line is a guide for the eye. The theoretical normalized cavity coupling is plot in solid blue for p polarization and in orange for s polarization. The grey area gives the boundaries for coupled cavity operation. The central blue area close to zero degrees corresponds to the angular range where mode-beating was observed.